Electron discharge device



May 15, 1951 F, E, GEHRKE 2,553,184

ELECTRON DISCHARGE DEVICE Filed Feb. l, 1947 INVENTOR.

Patented May 15, (1951 ELECTRON DISCHARGE DEVICE Forrest E. Gehrke, KewGardens, N. Y., assignor to Sylvania Electric Products, Inc., acorporation of Massachusetts Application February 1, 1947, Serial No.725,770

11 Claims. l

This invention relates to electron discharge devices, and moreparticularly to grid-controlled gaseous electron discharge tubes such asthose commonly referred to as thyratrons. An object is to improve theconstruction of electron discharge devices, especially of thyratrons,for more reliable and uniform performance. To this end the preferredembodiment of the invention includes an anode, a relatively long andslender cathode, and a so-called solid grid between anode and cathode,the grid having a conductive wall with a slot extending generallytransverse to the length of the cathode. This construction assuresuniform performance among the thyratrons assembled with like parts, eventhough the positioning of the slot in the solid grid may vary somewhatin its relationship to the cathode and to the anode.

The invention and further objects and features of novelty will .bebetter understood from the following detailed discussion of twoembodiments thereof, in contrast to a prior art construction.

In the drawings:

Fig. 1 represents a fragmentary View of a prior art solid-gridthyratron, showing the grid and cathode in projection.

Fig. 2 is a fragmentary transverse section of the electrodes in thatprior art thyratron.

Fig. 3 is a similar transverse section through the prior art thyratron,showing the elements imperfectly assembled in a manner to be expectedwith routine production methods.

Fig. 4 is an elevation similar to Fig. 1 showing the solid grid andcathode of a thyratron improved according to the present invention.

Fig. 5 is a transverse section through this embodiment of the improvedthyratron.

Fig. 6 is an elevation in detail of another embodiment of the thyratronimproved according to the present invention, the envelope being shown insection.

Fig. '7 is a transverse sectional view along the line I-'I in Fig. 6.

In the prior art it has become customary to use a so-called solid gridin thyratrons to control the discharge of electrons from cathode toanode. The helical wire or cage grids which are characteristic ofamplifier-type vacuum tubes are inadequate to provide the desiredcontrol so that a conductive wall, breached only to provide a dischargepath of limited cross-section, functions as the control grid. In Figs. 1to 3 a solid grid I0 is shown in its relation to a llamentary cathode l2and (Fig. 2) an anode I4. The aperture in the solid grid is showncentered about the line of centers from cathode and anode. Consequentlythe electrostatic field set up by control grid IE! (dashes representingequipotential lines) is symmetrical. Ordinarily the control grid Ill isoperated at a potential made negative with respect to cathode I2 whileanode i4 is considerably positive with respect to the cathode. Underthese conditions the long and narrow aperture in the solid grid iseffectively much narrower than it is physically, since the static fieldwhich the negative charge sets up reduces the gap through which thestatic lines of force between cathode and anode may act. When thepotential on the solid grid is sufficiently negative, the aperture is ineffect closed. When the negative potential is sufficiently reduced theeffective slot may open slightly, permitting gaseous conduction betweencathode and anode. In order that the thyratron of Fig. 2 may becontrolled by a relatively low negative potential an additional negativegrid in the form of negatively charged rods I6 is sometimes used.

It has been found that when tubes of this character are assembled withall reasonable care that there is a non-uniformity in performance; that,while it may be desired that the thyratrons Ishould fire when thenegative potential of solid grid I0 is between 2.1 and 2.7 volts minus,one thyratron may iire at minus 3 volts whereas another of the same typemay fire only when the negative potential on solid grid I0 is reduced tominus 1.2 volts.

Referring to Fig. 3 one explanation of this random performance is given.It is assumed in Fig. 3 that the relative spacing of the variouselements is the same as in Fig. 2 and that their dimensions are also thesame. It is of course difficult to maintain the aperture in numeroussolid grids IB to absolute uniformity, and slight variations in ringpotential among several thyratrons may be expected because of variationsin the width of this aperture. The length of the aperture is notcritical. In Fig. 3 the aperture in solid grid I0 is shifted laterallywith respect to the line of symmetry of the other elements. The resultof this difference from Fig. 2, even though it is caused solely by animproper positioning of the solid grid to the extent of half of its verynarrow aperture width, is not only to increase the distance betweencathode I2 and the center of the solid-grid aperture through which thedischarge must commence, but also to upset the symmetrical electrostaticiield pattern that should be set up by the charge on solid grid 3 l andby negative grid I6. It is believed that these factors are responsiblefor the serious variations in firing voltage among prior-art tubes of agiven type.

The result would be the same were solid grid I 0 assembled along theline of symmetry, with cathode l2 laterally displaced by an amount equalto so little as one half the width of aperture in the solid grid.A Thisprior-art aperture has been held to the width specified, within atolerance of 0.0005 inch, in an effort to achieve the performancespecified. Inaccuracies in the positioning of negative grid I6 arerelatively unimportant, it is believed, because of the relatively slightcontrol exerted thereby. The firing of thyratrons having two negativegrids is usually relatively independent of the negative voltage on thewide-apertured second grid near the anode. The variable potential orcontrol potential is applied to the first grid which is solid in thisinstance. Throughout this specication, where the terms rst grid orsecond grid are used, the terms refer to the order of the grids in thedirection from the cathode to anode, as is usual practice in theindustry.

From Fig. 3 it will be apparent that the positioning of a long andslender cathode in relation to a long and slender but generally parallelaperture in a solid grid is critical in that the positioning of both ofthese elements in accurate parallelism along the line of symmetry of thesecond grid and the anode results in a symmetrical field pattern whereasasymmetrical positioning produces a very different eld. Moreover., theeffective `distance between the cathode and the aperture is considerablychanged when one of these is shifted laterally from the line ofsymmetry. The length of electron path is believed to be a criticalfactor affecting the ring voltage of the solid grid.

The present invention aims at reducing the precision required inpositioning the solid grid laterally of the electron path, at enlargingthe allowable variating in aperture-width for reasonably uniform ringvoltage, and at more uniformly determining the firing voltage of thethyratrons into which the grids are incorporated. In Figs. 4 and 5 thereis shown diagrammatically, a solid grid i8, that is, a grid memberhaving a wall portion, such as of conductive sheet material, and havingapertures in the wall portion transverse (in projection) to cathode 20.In this construction, just as in the solid grid of Fig. 1, the width(rather than the length) of the individual aperture controls the firingvoltage. While three apertures are shown, they are spaced `sumciently sothat there is no important interaction among the elds of the three, andwhen the thyratron actually does re the resultant conduction is entirelythrough one of the apertures, the other two having no effect. When theemissivity of the cathode opposite one of the apertures decreases,another of the apertures becomes effective. The use of multipleapertures thus promotes long useful life of the thyratron withoutundesirably affecting its performance.

A salient distinction of the solid grid of Figs. 4 and 5 over that inFigs. 1 to 3, resides in the fact that the aperture or each of theapertures is not parallel to the long and slender cathode. Because ofthis construction and arrangement it now becomes relatively unimportantwhether the cathode is positioned perfectly opposite the center line ofthe aperture or not.v Because of the very low internal resistance-dropthere .is

low anode dissipation, and it is feasible to make the anode of smalldimensions. I have found that a rod will serve admirably, both from afunctional standpoint and for its adaptability to manufacturingpractices, for small thyratrons of relatively low current ratings. InFig. 5 rod 22 serves as the anode. A pair of connected rods 24 serve asthe second grid, as in the thyrat'ron of Figs. 2 and 3. The' length ofthe apertures in solid grid I8 is of the same order of magnitude as theseparation between rods 24, and this is made great enough to permit aproper breakdown current, Without increasing the width of slot in thesolid grid.

In Figs. 6 and 7 there is shown a second form of thyratron embodying theprinciple of Figs. 4 and 5 in that the solid grid has a long and narrowslot o't' parallel to the 1ong dimension of cathode 28, which is coatedwith electron emissive material at its center only as indicated at 30.Solid grid 26 is supported onthree rods two of which 32' are sealed intothe glass envelope 34 while the thirdv supporting rod 3B for solid grid26 is welded to a conductor 31 which extends through envelope 34. Thesecond grid 38, a desirable though not essential element of thethyratron, is in the form of a U-shaped rod Welded to a lead 40lextending through the glass envelope 34 parallel. to lead 31 of thefirstgrid. Anode rod 42 is welded to its lead 44 which is similarlysealed through enveope 34. Cathode l28 is connected to .lead 46 (Whichis sealed through envelope 34) by a tab 48. To avoid confusion two moreleads have been omitted from Fig. 6 which are normally provided toenergize a lamentary' heater (not shown) within tubular cathode 28. Bylimiting the emissive coating `3i! to the short length shown, it becomespossible to economize greatly in the amount of power required forheating the cathode. And because of the relatively great area of theindirectly hea-ted cathode in comparison te the lilamentary cathode,only one aperture for the firing current is regarded ample. I

rlfhe envelope 34 of the 4tube in- Figs. 6 and 7 is, as is usual,gas-filled. Cathode 28, anode 42, and the two rods of the second grid 38all project through top mica 50 and bottom mica 52 which hold them incorrect mutual spacing. Rods 32 and 3,6 of solid grid 26 also projectthrough a top and bottom mica to fix that grid in proper relation to theotherelectrodes. yConductors 31, 40, 44, and 46 as well .as the dummyrods 56 which supportv rods 32 .are all lap welded to the severalelectrodes and butt against the lower surface of bottom mica 52,-thereby preventing that bottom mica from Ashifting axially of thethyratron. The top mica is vheld in place lby staking the rods abovethat mica, and by welding getterstrip 54 to rods 32` and in lateralcontact withr the top `surface of mica 50.

The thyratron illustrated in Figs. 6 and '7 has flexible 'externalconductors intended for direct wiring into the circuit in which thethyratron is to be used. The external diameter of the glass envelope isapproximately 3/8 of an inch. The width of the slot in the solid grid isapproximately one millimeter -to -.00l inch tolerance, uand withstandard xed voltages on the anode and the second grid, the first soli-d`grid will control firing of a number 'of thyratrons to withinapproximately w0.37 `volt above or below minus 2.6 volts. This latitudeis due largely to variations in emission, .gas pressure and processing.The thyratron current is about l'25 ma. VWere .it not for the transversearrangement of the slot in the solid grid such uniform performance couldnot be realized. With a given standard of precision the constructiondescribed considerably improves the thyratrons forrperformance inaccordance with their specifications.

The use of multiple slots in the solid grid, made possible by the skewor transverse slot arrangement, is an additional feature enhancing theutility of this invention. Furthermore, the satisfactory performance ofa rod-shaped anode substantially parallel to the cathode is in largemeasure attributable to this form of solid grid. With a rod-like anode,there is merit in the transversely slotted grid even were the cathoderelatively broad.

While a speoic construction of thyratron has been discussed in detail,and certain of its features have been especially emphasized, it will berecognized that the invention possesses other features of utility, andis subject to other modifications than those shown. Thus, while it ispreferred to use the second grid, it might be considered satisfactory toomit the second grid and rely solely on the single solid grid forcontrol, and for determining the static voltage at which the tube wouldi-lre. Also, while the aperture of the solid grid appears in projectionperpendicular to the cathode, a substantial angle although less than 90would also serve the purpose. Therefore the specic disclosure should beconsidered merely illustrative, and not by way of limitation.

What is claimed is:

1. An electron discharge device having, within an envelope containing anionizable gas, a relatively long and slender cathode, and anode, and asolid grid between said cathode and said anode, said solid grid having arelatively long and narrow aperture having its length arranged at anangle to said cathode.

2. A thyratron having, within an envelope containing an ionizable gas, arelatively long and narrow cathode, and anode, a solid grid positionedbetween said cathode and anode, said solid grid having an aperture skewto said cathode.

3. An electron discharge device including, within an envelope containingan ionizable gas, a straight lamentary cathode, a rod-like anodeparallel to said cathode and a sheet metal grid interposed betweencathode and anode, said anode and cathode being parallel to each other,said grid being provided with a long and narrow aperture substantiallytransverse to the plane of said cathode and anode.

4. In a thyratron having, within an envelope containing an ionizablegas, a relatively long and slender anode and a relatively long andslender cathode lying in a common plane with said anode, a grid betweensaid cathode and said anode in the form of a conductive wall having anaperture which is long in the direction perpendicular to the plane ofsaid cathode and said anode, and which is narrow in its other dimension.

5. A thyratron having, within an envelope containing an ionizable gas, along and slender cathode, a long and slender anode parallel to saidcathode, a conductive-wall grid between said cathode and said anodehaving plural rectangular apertures narrow in the direction of the planecommon to said cathode and said anode, and widely spaced apart incomparison to their width, said apertures being long in the directiontransverse to said plane all of said apertures intersecting said commonplane.

6. A thyratron according to claim 5 including a second control gridhaving a long and wide aperture the length of which is approximatelycoextensive with the eiective length of said cathode and said anode, andwhich is positioned between said rst-mentioned grid and said anode.

7. An electron discharge device having, within an envelope containing anionizable gas, a long and slender cathode, a grid of conductive sheetmaterial effectively constituting an electrostatic shield around saidcathode, and a rod-like anode external of said solid grid, said gridhaving an aperture at a substantial angle to said cathode.

8. An electron discharge device having, within an envelope' containingan ionizable gas, a long and slender cathode, a grid of conductive sheetmaterial effectively constituting an electrostatic shield around saidcathode, a rod-like anode external of said solid grid, said grid havingan aperture at a substantial angle to said cathode, and a second grid inthe form of a U, the sides of which are substantially parallel to saidcathode and which are spaced apart approximately the length of theapertures in said rst-mentioned grid.

9. A thyratron having, within an envelope containing an ionizable gas, acathode, a long narrow rod like anode parallel thereto, and a solid gridfor effectively separating said anode and said cathode and provided witha number of apertures at a substantial angle to said anode.

10. A thyratron having, within an envelope containing an ionizable gas,an electron emitting element, an electron receiving element, saidelements being relatively long and narrow and lying in a common plane, asolid grid for isolating said elements electrostatically duringnon-conducting intervals, said grid having a long and narrow apertureskew to said long and narrow element and intersecting said common planeto provide a current path at other intervals.

11. A thyratron according to claim 10 wherein said aperture is close tosaid emitting element.

FORREST E. GEHRKE.

REFERENCES CITED The following references are of record in the lle ofthis patent:

UNITED STATES PATENTS Number Name Date 2,080,235 Smith May 11, 19372,201,880 Bruce May 21, 1940 2,296,324 Bahls Sept. 22, 1942 2,391,967Hecht et al Jan. 1, 1946 2,404,920 Overbeek July 30', 1946 FOREIGNPATENTS Number Country Date 475,106 Great Britain Nov. 12, 1937

